Confidentiality represents a core principle of research ethics and forms a standard practice in social research. However, what should a researcher do if they learn about illegal activities or harm during the research process? Few systematic studies consider researchers’ attitudes and reactions in such situations. This paper analyzes this issue on the basis of in-depth interviews with Polish sociologists and anthropologists who conduct qualitative research with vulnerable participants. It discusses the experiences and opinions of researchers concerning the maintenance or breaking (...) of confidentiality in the context of knowledge about illegal activities and harm. It also examines the ways in which the researchers justified their decisions. Most of my interviewees respected confidentiality in spite of knowledge of crime or harm, and referred to their epistemological perspectives regarding the role of the researcher, implicit consequentialist ethical reasoning and personal values. Where researchers did break confidentiality, this owed to their personal values and willingness to protect their informants, especially in cases of minor levels of harm as opposed to serious crime. Therefore, their experiences indicate the failure of both obligatory unconditional assurances of confidentiality and the requirement for researchers to assure confidentiality to the extent permitted by law. I argue that researchers do not need constrictive and potentially punitive rules about confidentiality, but rather sensitizing frameworks about how to contemplate and anticipate the many complexities and moral shadings of situations in the field. (shrink)